Half-Titration Point: Reading pKa From a Titration Curve

The half-titration point (also called the half-equivalence point) is the point on a titration curve where exactly half of the weak acid has been neutralized, which is halfway to the equivalence point by volume. There the acid and its conjugate base are present in equal amounts, so the measured pH equals the acid’s pKa. Reading the pH at that volume is the standard way to determine pKa from a titration curve.

Short answer

Find the equivalence point on the curve, take half of the titrant volume used to reach it, and read the pH at that volume. For a weak acid titrated with a strong base, that pH is the pKa. The rest of this page covers when that shortcut can be trusted, which is the part most summaries skip.

Definition and scope

The half-equivalence point is described as the point “halfway to the equivalence point”: the weak acid HA is half neutralized, so [HA] = [A^–]. Because the base-to-acid ratio is 1, pH = pKa [1][2]. The term refers to a volume, half of the titrant needed to reach equivalence, and not to the middle of the pH rise.

Before the equivalence point, the HA/A^– mixture is a buffer that resists rapid pH change [1]. This buffering connects the half-equivalence point to buffer work in the lab: see buffer and solution preparation for the Henderson-Hasselbalch relationship used to prepare buffers.

How to determine pKa from a titration curve

A lab manual for identifying an unknown weak acid states that the pKa can be determined by estimating the pH at the volume point halfway to the equivalence point, and that this pH can be used to identify the unknown acid [3]. The reading sequence is:

1. Locate the equivalence point. The lab manual does this graphically: fit best-fit lines to the flat top and bottom portions of the curve, draw a vertical line between them, and take the middle of that vertical line [3]. Potentiometry, meaning tracking pH with an electrode, is described as the standard tool for determining equivalence points [2].
2. Halve the volume. Divide the titrant volume at equivalence by two.
3. Read the pH at that volume. That value is the pKa estimate [2][3].

As a purely hypothetical illustration of the arithmetic: if the equivalence point fell at 20.0 mL of titrant, the half-equivalence volume would be 10.0 mL, and the pH read from the curve at 10.0 mL would be the pKa estimate.

The pH reading is only as good as the electrode. The same lab manual calibrates the pH electrode with two buffers, pH 4 and pH 7, before titrating [3]; the pH meter guide and the page on calibration intervals cover the instrument side. For the concentration arithmetic behind the titrant, see molarity and solution calculations.

When pH = pKa at the midpoint stops being reliable

The shortcut depends on being able to place the equivalence point. Use this checklist before trusting the reading:

Check	Why it matters	Source
Is the equivalence point clearly detectable?	Extremely weak acids give short jumps. Boric acid (pKa 9.3) is cited as hard to detect reliably.	[2]
Is the acid weak enough to leave a buffered region, but not so weak that the jump is lost?	For a weaker acid (larger pKa), the pH change around the equivalence point is much less pronounced.	[1]
Is the solution concentrated enough?	Chem1 advises keeping titrant and analyte concentrations above 10-3 M for differential plots.	[2]
Is the acid polyprotic?	There are as many equivalence points as replaceable hydrogens; they separate cleanly only when successive Ka values differ by several orders of magnitude. The most acidic proton is titrated first.	[1][2]
Is the final step obscured?	For phosphoric acid, the third equivalence point is obscured by water buffering.	[2]

For a polyprotic acid, each equivalence point belongs to a different proton, so identify which step you are reading before assigning a pKa; the sources cited here do not give a half-equivalence rule for polyprotic curves.

Where it matters for labs

Because the reading comes from a potentiometric curve, the quality of the pKa depends on a calibrated electrode and a well-prepared titrant. Indicators are a coarser alternative: an indicator colour change spans about 1.5 to 2 pH units [2], while the electrode records the full curve. Teaching and analytical labs that use half-equivalence readings to identify unknown weak acids [3] therefore depend on two-point electrode calibration and accurate titrant concentration. See also the page on the analytical calibration curve for a different use of the word "curve" in analytical work.

Common misunderstandings

• Reading the pKa at the equivalence point. For a weak acid with a strong base, the pH at equivalence is greater than 7.00 because A^– hydrolyzes [1]. It is not the pKa.
• Treating endpoint and equivalence point as the same thing. The lab manual states they are not synonymous [3]. The endpoint is what you observe; the equivalence point is the stoichiometric one.
• Taking the midpoint of the pH rise instead of the midpoint of the volume. The half-titration point is defined by volume, as above.
• Expecting a single equivalence point from every acid. Polyprotic acids have one per replaceable hydrogen [2].

FAQ

What is the half titration point?

It is the point halfway to the equivalence point, where half the weak acid is neutralized and [HA] = [A^–]. The pH there equals the pKa [1][2].

How do you determine pKa from a titration curve?

Locate the equivalence point, halve the titrant volume, and read the pH at that volume; that pH is the pKa [2][3].

Is the half-equivalence point the same as the endpoint?

No. The half-equivalence point is half the volume to equivalence. The endpoint and equivalence point are also not synonymous [3].

Sources

Sources checked 2026-10-29. All three are open LibreTexts textbook and lab manual pages.

1. [1] LibreTexts, University of Toronto, Chemistry: Physical Principles, Acid-Base Titrations: https://chem.libretexts.org/Courses/University_of_Toronto/Chemistry:_Physical_Principles/09:_Buffers_Titrations_and_Solubility/9.03:_Acid-Base_Titrations
2. [2] LibreTexts, Chem1 (Lower), Acid-Base Equilibria, 13.5 Acid-Base Titration: https://chem.libretexts.org/Bookshelves/General_Chemistry/Chem1_(Lower)/13:_Acid-Base_Equilibria/13.05:_Acid_Base_Titration
3. [3] LibreTexts, General Chemistry Labs II, Using a Titration Curve for Identifying an Unknown Weak Acid: https://chem.libretexts.org/Ancillary_Materials/Laboratory_Experiments/Wet_Lab_Experiments/General_Chemistry_Labs/General_Chemistry_Labs_II/16.2:_Using_a_Titration_Curve_for_Identifying_an_Unknown_Weak_Acid